Electronic devices whose physical security must be guaranteed, e.g., cryptographic and weapon-related devices, warrant special precautions against unauthorized physical access and undetected modification. Geographically dispersed manufacturing and assembly processes frequently result in many opportunities for unauthorized access, and after they are assembled and in operation, secure devices continue to require careful monitoring against unauthorized physical access. Physical access can result in circuit modification or an attachment of a monitoring device that can result in undetectable information leaks, a means to shut down a device's operation at will, and an opportunity to insert erroneous or misleading information into a secure information flow by an unauthorized entity.
Tamper evident techniques are less costly to implement than tamper prevention techniques, but both techniques are sometimes used concurrently. A tamper evident technique attempts to make a physical access to a device nearly impossible without leaving evidence of the access. Attractive tamper evident techniques provide an evidence of a tampering that is easily detectable without using expensive equipment or lengthy procedures while making the tampering impossible to accomplish without leaving such evidence. A tamper evident technique is even more attractive if a tampering entity remains oblivious to any evidence that his or her tampering creates, leaving several options open for security enforcement personnel to manage a tampering incident.
Some tamper evident techniques that are applied to electronic devices use mechanical enclosures and/or seals which are difficult to bypass without leaving evidence of a tampering event. However, these techniques increase weight which can be a factor in some implementations and they may require microscopic examination of an enclosure or a seal to detect a tampering. Also, they are less effective in an environment where sophisticated tools may be available to perform a tampering, subsequent to a physical interception of a device during manufacture, transportation, or storage, for example.